The present invention relates to an apparatus for electrooptically focusing the photographic objective in a mirror reflex camera.
Such an apparatus is known in the form of focusing detectors scanning an image segment in a conjugated image plane, for instance by using one or more photodiode lines or by spatial frequency filtering using a moving grating, a maximum of the amplitude of this spatial frequency occurring with maximum focus.
U.S. Pat. No. 3,953,728, the disclosure of which is incorporated herein, further discloses adjusting for a phase null in lieu of an amplitude maximum by obtaining two sets of data with different phases from opposite pupil regions.
As in U.S. Pat. No. 3,953,728, the disclosures of U.S. Pat. Nos. 3,856,400 and 3,856,401 are incorporated herein to show the state of the art of apparatus for determining the relative position of the plane of maximum amplitude and the gratings useful in the present invention while U.S. Pat. No. 3,781,110 is incorporated to show the fundamentals relating to the use of push-pull signals.
U.S. Pat. No. 3,953,728 defines an apparatus having an imaging system with at least one optically effective grating where a photoelectric receiver system is employed to determine the relative position of the plane of maximum amplitude of a spatial frequency component in the image of an object. The imaging system has an entrance pupil and this entrance pupil has regions which are traversed by light fluxes emanating from the object. The imaging system forms an image of the object in a plurality of intermediate image planes and the grating displays a given spatial frequency component and is mounted in one of the intermediate image planes for receiving the image and for modulating the light fluxes. The photoelectric receiver system consists of at least two photoelectric detectors responsive to the light fluxes passing through the grating and the detectors generate electrical push-pull signals for use in governing control means. A focusing screen is mounted in the first of the intermediate image planes and has an optical measurement wedge for dividing the focusing screen into two areas. One of the areas is for subjectively observing and focusing the object and the other area has a given size for transmitting and further processing the light fluxes which are selected from one of the regions of the entrance pupil. The grating is located in a second of the intermediate image planes and modulates the light fluxes traversing the selected region while splitting the light fluxes into at least two components. The components are directed to the photoelectric detectors. A pentaprism is positioned between the focusing screen and the grating to fold the beam path. The photoelectric detectors generate signals which control the focusing of the photographic objective of the mirror reflex camera.
When an apparatus, as defined in U.S. Pat. No. 3,953,728, is used in a single lens reflex camera it is known to project a segment of the focusing window of the focusing screen onto the scanning grating, to mount a fully reflecting surface constructed as a concave mirror to the pentaprism at the side facing the ocular, and to keep the central area of this reflecting surface nonreflecting. This nonreflecting area permits observing the focusing window using the viewer ocular.
However, this prior art system suffers from the drawback that the mirror surface next to the necessarily nonreflecting area is too small to reflect light from large aperture angles to the scanning grating. On the other hand, where interchangeable small-aparture objectives are concerned, it is nevertheless necessary to coat the central area previously termed nonreflecting at least to be partly reflecting in order to apply sufficient light for these objectives and to the scanning grating and the subsequent photoelectric detectors.